This invention relates to micromachined gyros.
A surface micromachined gyro has a planar body (or a number of bodies) suspended with anchors and flexures over and parallel to an underlying substrate. The body is dithered along a dither axis in a plane parallel to the substrate and perpendicular to a sensitive axis that can be in the plane of the body or perpendicular to the body and to the substrate. As is generally known, rotation by the body about the sensitive axis causes the body move along a Coriolis axis, which is mutually orthogonal to the dither axis and the sensitive axis. This motion can be sensed to derive a signal that indicates the angular velocity of the rotation.
Because of mechanical imperfections in the body and in the flexures, a suspended mass will typically not be perfectly parallel to the substrate, and the dither and sensitive will typically not be perfectly orthogonal. Consequently, when the body is dithered, an interference signal, referred to as the quadrature signal, is induced by the dithering motion itself. This quadrature signal, which is unrelated to the rotation to be sensed, interferes with the desired signal relating to the rotation. The quadrature signal (a) is proportional to the acceleration in the dither direction with a constant of proportionality indicative of the mechanical misalignment; (b) has the same frequency as the dither frequency; and (c) is 90° out of phase with the dither velocity, unlike the Coriolis signal which is in phase with the velocity. Because of this 90° phase difference, the quadrature signal can be partially rejected with a phase-sensitive detector. The effectiveness of such rejection, however, depends on how precise the phase relationships are maintained in the electronics.